The prior art method of sensing paper in a printer is to use mechanical flags in the paper path. Paper passing along the paper path causes the flags to move, triggering a signal. The signal indicates the presence of the paper. One drawback to the use of these flags occurs when the flags are used to monitor paper entering or exiting rollers. The mechanical flags must be located some distance away from the rollers so that neither the flags nor the rollers interfere with the other's operation. The flags therefore cannot be used to determine the precise time at which the paper enters or exits the rollers.
An inversion process is an example of when a more accurate determination of this time is important. The inversion process is used to discharge a printed page with the printed side down. Discharging pages with the printed side down allows a sequence of pages to be printed and discharged into a stack with the first page of the sequence being the bottom page of the stack.
Inverting a page must be performed quickly as another page may be following closely behind the first printed page. In order to take full advantage of the time allowed for inverting a page, it is desirable to start the inverting process as soon as possible. Providing a means for precisely detecting the exit time of a page from a roller allows the inverting process to begin as soon as the page leaves the rollers immediately preceding the inverter.
Accordingly, given the foregoing background relating to sensing paper entry and exit with respect to rollers, an object of the present invention is to provide a system and method for detecting more precisely when a workpiece is received by or released from a pair of rollers.